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One of main mission of KSTAR is to develop long pulse operation capability relevant to the production of fusion energy. After ITER has decided to begin with full metal wall configuration, KSTAR has planned a major upgrade to tungsten first wall similar to JET ITER-Like Wall (coatings and bulk tungsten plasma-facing components). To accomplish the upgrade, tungsten bonding technology has been developed and tested. Since leading edges of each castellation structure have to be protected, shaping of tungsten blocks has been studied by ANSYS simulation and the miniaturized castellation has been exposed to Ohmic plasma to confirm the simulation results. It is found that shaped castellation block has more heat handling capability than that of conventional block. For more dedicated experiments, a multipurpose castellation block is fabricated and exposed to Ohmic, Land H-mode plasmas and observed by IR camera from the top. During the fabrication and assembly of the blocks, leading edges caused by "naturally misaligned" blocks due to engineering limit with a maximum level up to 0.5 mm have been observed, which has to be minimized for future fusion machine.

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The central solenoid (CS) coil system of the KSTAR device consists of four pairs of coils with up-down symmetry and a set of support structure. The CS coil system should endure various loads including thermal and electromagnetic load during operation. One of the most important roles of support structure is to preserve the structural stability of coil system from vertical forces by applying the pre-compression on the coil stack. The main assembly procedure of the coil stack is as follows: 1) coil subassembly and stacking, 2) structure assembly, 3) preloading process by heating shells in the support structure, and 4) measurement on the final dimension. The special jigs and heating system for the preloading were designed to apply pre-compression on the coil stack during the assembly at room temperature. Moreover, a few tens of thermocouples and strain gauges were installed for measuring temperature and thermal expansion of preloading structures. Since the final assembly of the CS coil was successfully completed with about 7.3 MN of preloading in 2006, the CS coil system has been experienced four thermal cycles from 300 K to 5 K until 2011. This paper describes the main results of applying the pre-compression on the coil stack and mechanical behaviors of preloading structures during thermal cycles.